Process for electrodeposition on aluminum and aluminum alloys



Patented Jan. 3, i939 UNITED STATES PATENT OFFICE UMINUM Joachim Korpiun, Berlin-Grunewald, Germany, assignor to Schering-Kahlbanm A. G., Berlin, Germany, a corporation of Germany No Drawing. Application November 18, 1936, Serial No. 111,389. In Germany November 19,

1935 14 Claims.

It is known to try to obviate the difliculties met with in the production of electro-galvanic deposits on articles of aluminum and aluminum alloys with the aid of special pickling solutions with which the aluminum or its alloys are treated prior to the galvanization. These pickling solutions are principally divided into two groups. In connection with one of the groups it is intended to obtain adherent electro-galvanic deposits on aluminum by using acids or acid mixtures, with or without admixture of metallic salts, as pickling solutions. These solutions produce an adhesive surface for the deposits to be applied galvanically in most cases by roughing up the aluminum surface to a certain extent, thereby allowing of an improved adhesion of the metallic coating or deposit to the ground metal. These pickling solutions are mostly recommended for the preparatory treatment of the nickel-plating process proper. Also a process has become known in which an intensive preparatory treatment of the aluminum by means of acid mixtures is used as a preliminary step for silver-plating.

These processes do not, in most cases, wholly satisfy, because they roughen up the metal rather excessively and render the bumng process, i. e. providing a brilliant polish on the galvanized articles, more difiicult.

Moreover, this process of roughing up is not adapted for subsequent plating in cyanide baths.

The other group of the pickling'solutions for the preparatory treatment of the aluminum or its alloys prior to the galvanizing process, include the alkaline pickling solutions of which chiefly zincate dipping solutions are being used. The eiiect of the latter principally consists therein, that a firmly adhering coating of zinc is deposited upon the aluminum and subsequently the desired metallic deposit upon said zinc coating in an appropriate manner, whereby a better adhesiveness of the electro-galvanic deposits is ensured than if the latter were applied directly upon the aluminum. The alkaline zincate dipping solution is recommended not only for nickelplating aluminum, but also for the galvanic treatment of the aluminum in alkaline or cyanide baths. Whilst in connection with nickelplating aluminum, preparatorily treated with zincate dipping solution, difliculties are met with, for the obviation of which remedies have been indicated latterly, no method has become known yet, according to which the difliculties could be removed, which are met with in connection with the galvanization of aluminum goods, preparatorily treated with zincate dipping solution, in

alkaline, particularly in cyanide baths. It has been ascertained in practical work, that the attempts to obtain firmly adhering deposits, are characterized by a considerable amount of uncertainty. If, for instance, the aluminum has been preparatorily treated with zincate solutions, it will be altogether impossible to silver-plate the former.

The present application, now elucidates a process, by means of which the possibility of obtaining properly adhering deposits, particularly in alkaline baths, will be rendered practically certain. According to, this process the aluminum articles are, after the usual degreasing process, treated with a zincate solution or solution to which copper salts have been added.

It has, it is true, already been suggested to add copper salts to zincate solutions for the purpose of preparatorily treating aluminum prior to galvanizing the latter, but the coatings produced on the aluminum by means of this dipping process, according to said process are removed again by means of nitric acid before electro-plating takes place. Aluminum thus preparatorily treated, may be adapted for being nickel-plated, but not for galvanization in alkaline plating baths.

The composition of the dipping solutions adapted for the present invention for preparatorily treating the aluminum prior to the subsequent galvanization in cyanide baths is not a matter of indifierence. The eificiency of the dipping solution rather depends upon its content of free caustic potash, zinc and copper. It has, as a matter of fact, been ascertained, that the indicated aim would best be attained by means of dipping solutions, the content of free alkali of which is about 3 to 15 times normal, whilst the content of zinc should correspond at least to simple normality. The copper content of these solutions is then to be kept within the limits of from about 1.5 to 8.0 g. of copper per liter. In connection herewith it is of advantage, if the copper is present wholly or for the greatest part in univalent form. This will be attained, if the copper is added in the form of potassium copper cyanide or other soluble cuprous salts. In order to retard the oxidation of the univalent copper salt by the atmospheric oxygen, reducing agents, such as sodium-sulphite and/or similarly acting substances are added to the dipping solutions.

The articles preparatorily treated with the dipping solution can be galvanized directly in alkaline or cyanide baths. With certain alloys, par ticularly if they contain copper, and properly burnished articles, it often proves suitable to remove the deposit formed by the dipping solution, by means of oxidizing agents, such as nitric acid or acid mixtures containing nitric acid and to repeat subsequently the treatment with the cuprous zincate dipping solution. This process can, if necessary, be repeated a few times.

It has further proved advantageous to add to the baths, instead for the galvanization of .the aluminum articles treated according to the present invention, alkali carbonate and/or alkaline salts of fatty acids and/or alkaline salts of inorganic'acids. As fatty acids will have to be considered in this case, above all, acid with a low molecular weight, such as formic acid, acetic acid, propionic acid, etc., as inorganic acids, phosphoric acid, boric acid, etc.

A dipping solution for the preparatory treatment of the aluminum, according to the present invention, is composed, e. g. as follows:

Water liter 1 Caustic soda g 400 Zinc oxide g 80 Copper (e. g. in form of potassium-copper cyanide) g Sodium sulphite g 25 Depending upon the kind of article or of the alloy, the article is immersed in this solution for 7$ to 5 minutes subsequently rinsed and placed into the galvanic bath. If necessary, the removal of the deposit, described in the foregoing, by means of nitric acid, can be effected and the treatment by means of the clipping solution repeated.

According to the described process ,it will be possible to obtain perfectly adhering deposits at temperatures up to about 120 in copper, brass. cadmium, silver, gold baths and the like. The deposits allow of being polished by means of the bufllng wheel or also with the aid of the steel, without causing the well-known blisters. They also resist any deformation of the aluminum as e. g. by sharp bends, hammering and the like without causing thereby a detachment of the deposit. I

If the articles, treated in accordance with the present invention, should be subjected to temperatures above 120 C., it may happen, that the adhesive resistance of the electro-galvanic deposits, such as coatings of brass, copper or silver, decreases, which is rendered obvious by blistering and scaling.

In this case it is advisable, to add to the cyanide baths serving for galvanizing the preparatorily treated aluminum parts, substances, acting as acid, the dissociation constant of which does not exceed l.0-10- For this purpose boric acid, phenols, substituted phenols or the like will have to be considered. In this case it concerns such substances, the degree of acidity of which differs only very little from that of hydrocyanic acid, if at all. 1

Example To an ordinary copper bath prepared with potassium-copper-cyanide, 10 to 20 g. of pulverized boric acid are added and the copper plating process carried through in known manner.

For the purpose of improving the adhesive resistance of the deposits at higher temperatures, the galvanized articles of aluminum are, according to the present invention, after having been copper-plated, brass-plated and the like, treated for some time with weakly acid solutions. Aqueous solutions containing 1 to 5% of acid may be used. The treatment can consist in boiling the galvanized aluminum articles in a 2-5% solution of boric acid or in a solution of tartaric acid or the like.

It will generally be sumcient, if the duration of the subsequent t eatment extends over 5 to 10 minutes. The tre tmcnt in heated or boiling solutions is, however, not absolutely required. The same satisfactory effect can, however, be ensured by using said solutions at room temperatur'e in connection with which the duration of treatment should be suitably prolonged. It has been ascertained that in these circumstances a treatment extending over 10 to 30 minutes, will produce the desired eflect.

The nature and concentration of the acid solutions used for the after-treatment is not of a decisive importance with regard to the effect of this treatment, as long as the galvanized articles are not beingattacked thereby or otherwise deleteriously influenced.

By an examination of the articles of aluminum or aluminum alloy, which were preparatorily treated according to the described process and galvanized in due consideration of the detailed measures and subjected to the after-treatment, it has been ascertained, that these articles sustain a temperature of 350-400 for a prolonged period. With nickel deposits, applied to such copperor brass-plated sheet aluminum, the heating could be continued until the aluminum began to melt, without causing the nickel deposit to peel oil.

It has also been ascertained, that in many cases faultless articles are obtained. if they are subjected either to the after-treatment in baths only, or if they are galvanized in baths, to which the weakly acid substances have been added. A combination of both processes has, however, proved to represent the most advantageous method.

Of course, various modifications and changes in the reaction conditions, etc., may be made by those skilled in the art in accordance with the principles set forth herein and in the claims annexed hereto.

What I claim is:-

1. Process for the production of electroplated deposits on articles of aluminum or aluminum alloys after currentless preparatory treatment by means of zincate dipping solution containing caustic alkali, characterized in that for the preparatory treatment a zincate dipping solution is used which contains univalent copper salts and that the produced intermediate coating remains on the aluminum after the preparatory treatment, whereupon the electroplating is effected.

2. Process according to claim 1, characterized in that the zincate solution contains an amount of caustic alkali such that the solution is about 3-45 times normal.

3. Process according to claim 1, characterized in that the content of zinc of the zincate solution is at least 1/1 normal.

4. Process according to claim 1, characterized in that the content of copper of the zincate solution amounts to about 1.5-8.0 g. per liter.

5. Process according to claim 1, characterized in that reducing agents are added to the zincate solution, said agents acting to retard oxidation of said copper salts by atmospheric oxygen.

6. Process according to claim 1, characterized in that the deposit caused to appear on the aluminum on dipping, is removed by means of oxidizing agents of such character as to be capable or removing said deposit and produced anew as a result of a repeated dipping into the zincate solution.

'7. Process according to claim 1, characterized in that the deposit produced on the aluminum as a result of the dipping, is removed by means of an oxidizing agent taken from the class consisting of nitric acid and acid mixtures containing nitric acid and produced anew by repeating the dipping operation into the zincate solution.

8. Process according to claim 1, characterized in that to the alkaline baths used for electroplating purposes, substances acting as acid, are added, the dissociation constant of which does not exceed the value of 110- 9. Process according to claim 1, characterized in that said electroplating is accomplished in cyanide baths and to the cyanide baths alkali carbonates are added.

10. Process according to claim 1, characterized in that the electroplated aluminum articles are subjected to an after-treatment with a weak acid solution, the concentration thereof being such that it does not attack the electroplated articles or only inconsiderably.

11. Process according to claim 1, characterized in that to the alkaline baths, at least one alkaline salt of an acid selected from the class consisting of phosphoric acid, boric acid and fatty acid containing at most three carbon atoms is added.

12. Process according to claim 1, characterized in that the electroplated aluminum articles are subjected to an after-treatment with a solution of an acid taken from the class consisting of boric and tartaric acids. I

13. Process according to claim 1, characterized in that the electroplated aluminum articles are subjected to an after-treatment with a solution of an acid, the concentration thereof being such that it does not attack the electroplated articles or only inconsiderably, said after-treatment being conducted at a temperature up to and including the boiling point of said solution.

14. Process according to claim 1, characterized in that the electroplated aluminum articles are subjected to an after-treatment with a solution of an acid, the concentration thereof being such that it does not attack the electroplated articles or only inconsiderably, said solution containing 1-5% oi said acid in aqueous medium.

JOACHIM KORPIUN. 

